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Science

Why we need chemicals

Energy changes in reactions

When a chemical reaction occurs, energy is transferred to, or from, the surroundings - and there is often a temperature change. For example, when a bonfire burns it transfers heat energy to the surroundings. Objects near a bonfire become warmer. The temperature rise can be measured with a thermometer.

Exothermic reactions

These are reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:

  • Burning
  • Neutralisation reactions between acids and alkalis
  • The reaction between water and calcium oxide

Endothermic reactions

These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can also be detected using a thermometer. Some examples of endothermic reactions are:

  • Electrolysis [electrolysis: Electrolysis is the decomposition (separation or break-down) of a compound using an electric current. ]
  • The reaction between ethanoic acid and sodium carbonate
  • The thermal decomposition [thermal decomposition : A reaction in which substances are broken down by heat. ] of calcium carbonate in a blast furnace

The animation shows an exothermic reaction between sodium hydroxide and hydrochloric acid, and an endothermic reaction between sodium carbonate and ethanoic acid.

In industry it is important for scientists to be able to predict if a reaction is going to be endothermic or exothermic. The main reasons for this are:

  • As energy is given out in exothermic reactions it could be harnessed to be used elsewhere.
  • Endothermic reactions require energy, therefore fuel is needed which costs money.
  • An increase in temperature makes chemical reactions go faster. So, if the reaction is exothermic (giving out heat energy) the rate of reaction may increase to an unsafe point causing an explosion.

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